Power consumption of modern computers has increased as a result of increased operating frequencies. Power consumption requirements of various components, e.g., microprocessors, within a modern computer system need to be met in order to sustain these increased operating frequencies. Thus, providing power to components within a computer system in an accurate and predictable manner is of critical importance.
Ultra Large-Scale Integrated (xe2x80x9cULSIxe2x80x9d) and Very Large-Scale Integrated (xe2x80x9cVLSIxe2x80x9d) circuits have strict requirements on signal integrity. In ULSI and VLSI circuits, transistors may switch very rapidly and draw significant amounts of current. Thus, there is a relative short amount of time during which a ULSI or VLSI circuit dissipates significant amounts of current. With increasing transistor densities on ULSI and VLSI circuits, this current requirement may lead to potential power problems and various signal integrity issues.
To overcome such performance inhibitions, one or more decoupling capacitors are positioned near a transistor to store charge. When needed, these decoupling capacitors distribute this stored energy to the required transistors. In essence, decoupling capacitors act as local power supplies for particular discrete elements, e.g., transistors, etc. This is necessary because a power supply for a computer system component, such as a computer integrated circuit, typically resides at some distance from discrete elements on the integrated circuit. Therefore, a discrete element may not get power (via current) at the exact time it is required.
It follows that an important concern for designers is to ensure that transistors have enough decoupling capacitance. Referring to FIG. 1a, to verify that there is enough decoupling capacitance on an integrated circuit 10, a designer selects one of several areas 11 on the integrated circuit 10, chooses a particular signal in that area (not shown), and verifies that there is enough decoupling capacitance associated with that particular signal. However, this verification process does not guarantee that there is enough decoupling capacitance within a given distance of a particular transistor.
Such a guarantee is important because driver delay, i.e., transistor delay, depends on how quickly power is transferred from a power supply or a decoupling capacitor to a particular driver. Typically, power transfer from a power supply is slower than power transfer from a decoupling capacitor. Thus, it is imperative that a decoupling capacitor have enough decoupling capacitance within a relatively close proximity to a particular driver. The faster a driver receives power, the faster a signal driven by the driver may switch, effectively increasing the performance of the integrated circuit. FIG. 1b shows an expansion 12 of one of the areas 11 on the computer integrated circuit 10 shown in FIG. 1a. The expanded area 12 has decoupling capacitors 14 and transistors 16. Power supply lines 18 provide power to the decoupling capacitors 14 and transistors 16. The transistors 16, in turn, drive discrete other integrated circuit components (not shown) on the integrated circuit 10 by providing signals to them. The decoupling capacitors 14 store charge from the power supply lines 18 for distribution, as needed, to the transistors 16.
Thus, there is a need for a technique that verifies an amount of decoupling capacitance on an integrated circuit with respect to power needs of transistors on the integrated circuit.
According to one aspect of the present invention, a method for verifying decoupling capacitance on an integrated circuit having a transistor and a decoupling capacitor comprises determining a surface area of the decoupling capacitor, determining a surface area of the transistor, defining a surface area ratio as a ratio of the surface area of the decoupling capacitor to the surface area of the transistor, and comparing the surface area ratio to a predefined range, wherein the range is between a predefined lower value and a predefined upper value.
According to another aspect, a computer system comprises a memory and a processor, wherein instructions residing in the memory are executed on the processor for determining a ratio of a surface area of a decoupling capacitor to a surface area of a transistor and verifying decoupling capacitance on an integrated circuit using the ratio.
According to another aspect, an apparatus that verifies an amount of decoupling capacitance on a microprocessor comprises means for determining a surface area of a transistor and a surface area of a decoupling capacitor, means for comparing the surface area of the decoupling capacitor to the surface area of the transistor, and means for determining whether there is sufficient decoupling capacitance based on the comparison.
According to another aspect, a method for designing a microprocessor comprises determining an amount of decoupling capacitance required by the microprocessor, verifying the amount of decoupling capacitance on the microprocessor (where verifying the amount of decoupling capacitance comprises obtaining a ratio using surface area information of a decoupling capacitor and a transistor and comparing the ratio to a predefined lower value and an upper value), and redesigning the microprocessor based on the comparison.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.